1. Field of the Invention
The present invention relates to a catadioptric optical system and especially to a lens driving apparatus thereof.
2. Description of the Prior Art
A catadioptric optical system is often used to obtain a compact structure of a telephoto lens of long focal length for a camera.
The photographing lens employing a catadioptric optical system generally comprises lens groups containing two reflective surfaces arranged at the light entrance portion and at the middle portion of the lens barrel. The optical system is constituted of four parts, which are, as viewed in the direction of the incident light, the first dioptric system, the first catoptric system, the second catoptric system and the second dioptric system.
For focusing of the above-mentioned type of optical system there have been known and used various focusing methods. For example, the following three different methods have been used. The first is to move the optical system toward the object as a whole. The second is to change the distance between the two reflective surfaces. The third is to change the distance between the reflective surfaces and the second dioptric system while keeping the distance between the two reflective surfaces constant.
The first focusing method involves some drawbacks. As the whole optical system is moved for focusing, the amount of movement required for focusing becomes larger for the optical system having longer focal length. Consequently, this method needs a large and heavy driving mechanism. In addition, the position of the center of gravity changes in the extreme by focusing. This is against the desire for easy operation.
According to the second focusing method, the amount of movement of the optical system required for focusing is relatively small. However, the lens elements to be moved are large-diametered lens elements. Therefore, this focusing method also needs a large driving mechanism the use of which is undesirable with respect to easy operability. Furthermore, as the reflective surface is moved for which the highest precision is required, there is often caused eccentricity of the reflective surface by the movement. Thus, it is difficult to maintain the optical performance of the optical system.